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V.8 Modem DSP C Code

Description: implements the ITU modem Recommendation V.8 for the standard 2-wire telephone network. V.8 is used by data modems / fax machines developed since the early 1990s to exchange operational menus. The V.8 signals are the initial sequence exchanged following connect-to-line. They start with a modified version of the 2100Hz auto-answer signal for disabling echo-control devices. The operational menus are then exchanged using V.21 300b/s FSK modulation. The Calling channel uses 980Hz(Mark) and 1180Hz(Space) tones whilst the Answering channel uses 1650Hz(Mark) and 1850Hz(Space) keying tones. As part of the menu exchange in V.8, the highest common denominator modulation mode is chosen.
         The C code is suitable for any processor with an ANSI-compliant C compiler. It is designed especially for efficient operation on low-cost fixed-point DSP- and general-purpose micro-processors. All data memory is specified as 16-bit integer words. Multiplies are integer 16x16-bit with 32-bit products.
         The code is operated by one callable subroutine for initialisation and one for main-routine processing of input/output samples and data menus. The user is responsible for interfacing samples to hardware. The code is re-entrant to facilitate multi-channel operation. A comprehensive user's guide is provided with the code, together with example application files. Test results are available.

Interfaces: at the sample interface, the code can operate at 8.0kHz and 9.6kHz sample rates. At the data array interface, operational menus are loaded and accepted as octets in parameter groupings as per V.8. Start-stop bits are added and discarded in the V.8 code.
         At the control interface, the code has one 16-bit control word to sequence operation and to set operating parameters, and one diagnostic word to convey the V.8 handshake status and to provide a modulation mode listing of the V.8 negotiation result.

Performance:

  • SNR for 10-6 Error Rate at 300b/s (4kHz BW noise): 5.0dB Flat Channel

Processor Load:

Sample rate MIPS# Data memory (words) Prog memory (bytes)#
8.0kHz 6.3 114 3.5k (incl. V.21 code)
9.6kHz 7.5

- #example figures from compiling for the TI TMS320C5000(C55x) DSP processor

Availability: NOW - sale is under licence - integration support offered

 
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